The present invention relates to high-intensity discharge lamps such as a mercury lamp, natrium lamp and metal halide lamp (MHL), and more particularly, to a high-intensity discharge lamp in which the structure of its luminous tube is improved so as to improve luminous efficiency and color rendering.
Generally, lighting lamps of high brightness and long life are installed in street light fixtures and industrial work areas. Among such commercially available lamps, there are a high-intensity mercury lamp, a high-intensity natrium lamp and an MHL. Mercury lamps are the most widely used, and have a comparatively long lifetime. However, their luminous efficiency is somewhat poor and their luminous color is unappealing. Natrium lamps are best in view of luminous efficiency but their color rendering is somewhat poor. However, the MHL is better than the mercury lamp in view of luminous efficiency, and is best in view of color rendering. Accordingly, the use of MHL's is becoming more widespread. The cost of an MHL, however, is high and should be reduced in the near future. With MHLs being increasingly used, some prerequisites should be met. Particularly, in the field of interior design in which the illumination effects play an important role, such prerequisites can be satisfied when care is taken. Particularly, a small MHL which is used in the field of interior design should have low power consumption, high efficiency, high color rendering and a long lifetime. Here, the MHL which is chiefly used in an interior room will be described below.
FIG. 1 shows one example of a conventional MHL. Referring to FIG. 1, a pair of electrodes 2a and 2b are provided at both ends of a capsule-shaped luminous tube 1 made of quartz. Around each electrode is formed a zirconia heat-retaining layer 3. Also, luminous tube 1 is filled with predetermined rare gases, mercury and metal halide, and sealed. An outer tube 4 encloses luminous tube 1 and its accessories. Outer tube 4 is evacuated or sealed after being filled with nitrogen and inert gases. A socket connector 5 is provided on either end of outer tube 4, and is electrically connected with electrode 2a or 2b. Here, reference numeral 6 represents a getter which absorbs the remaining gas to increase the vacuum.
FIG. 2 is an enlarged view of the luminous tube of the lamp shown in FIG. 1. Referring to FIG. 2, luminous tube 1 is capsule-shaped and generally cylindrical. At the lengthwise ends of luminous tube 1 are provided electrodes 2a and 2b. From each electrode is drawn out a lead wire 7. To the respective lead wires 7 is installed a molybdenum thin plate 8 for maintaining a gas-tight seal. Also, as described above, on respective electrodes 2a and 2b is formed a heat-retaining layer 3, which prevents the temperature around the electrodes from decreasing.
However, in such a conventional MHL as above, when the lamp is in a state of illumination, the lower end of luminous tube 1 is cooled by convection of gas in the tube 1 and of nitrogen gas in outer tube 4, and becomes a minimum-temperature portion. Also, as a result of the temperature difference due to such convection, the arc created by discharge is bent upwards. Accordingly, the quartz luminous tube 1 is degraded by non-uniform local heating. On the other hand, the vapor pressure of the metal halide is varied depending upon the temperature of the minimum-temperature portion. Accordingly, condensation of the compounds in luminous tube 1 occurs by the cooling action in the lower end of luminous tube 1, resulting in insufficient vapor pressure, which lowers the efficiency of the lamp.